The electrochemical storage device described and claimed here takes the form of a high-temperature storage device which requires a minimum temperature of at least 100° C. as its operating temperature. In particular, the operating temperature is between 200° C. and 350° C., wherein the operating temperature must be of such a level that the solid electrolyte comprised by the electrochemical storage device is sufficiently ion-conductive for it to have the lowest possible internal resistance for the electrochemical storage device. Higher operating temperatures, for instance up to 500° C., are likewise conceivable. Typical electrochemical storage devices to which the invention relates are, for example, those based on sodium-nickel chloride cell (NaNiCl2 cells) or sodium-sulfur cell (NaS cells) technology. Various embodiments of these cells are known.
The structure of a conventional electrochemical storage device based on sodium-nickel chloride cell technology has a negative electrode which takes the form of the anode during discharging operation, the anode material of which assumes the form of liquid sodium at operating temperature. Said sodium typically fills part of the anode compartment during operation. The positive electrode during discharging operation, which takes the form of a cathode, has a cathode compartment which is at least in part filled by a suitable metal, for instance nickel, mixed with a likewise suitable salt, for instance NaCl, and for instance further suitable additives, such as for example AlCl3 or NaAlCl4. At the operating temperature of the electrochemical storage device, the mixture of these substances typically at least in part takes the form of a liquid electrolyte.
The anode compartment and cathode compartment are separated by a solid electrolyte which, as a ceramic separator, is permeable only to ions. The solid electrolyte is not intended to permit any mass exchange other than that involving the ions. Typical solid electrolytes which are used in sodium-nickel chloride cells comprise β-Al2O3 or β″-Al2O3 as the ceramic material. This permits a specific ion conductivity for Na+ ions which may be transported between the anode compartment and cathode compartment through the solid electrolyte depending on the potential gradient. The reactions which respectively proceed during charging operation or discharging operation are revealed for example by the following reaction equation (discharging operation from left to right; charging operation from right to left):2Na+NiCl2↔2NaCl+Ni
At the equilibrium state, a voltage of approx. 2.58 V may accordingly arise between the anode and cathode.
In order to enlarge the active surface of the anode material resting against the solid electrolyte which is available for said ion exchange, spring metal sheets are typically arranged in the anode compartment which in part rest against the solid electrolyte and in part rest against the wall surrounding said solid electrolyte. The spring metal sheets are on average only slightly spaced from the solid electrolyte such that, at operating temperatures at which the anode material is in the liquid phase, said anode material is moved by capillary action between the solid electrolyte and the spring metal sheet against the effect of gravity in such a manner that it is located above the filling level present in the anode compartment during operation. Even when the filling level of the electrochemical storage device with anode material is relatively low, it is in this respect possible to enlarge the active surface of the anode material brought into contact with the solid electrolyte. The solid electrolyte is consequently also sufficiently wetted above the filling level of the remaining anode material in the anode compartment, wherein the internal resistance of the storage device is accordingly reduced when the electrochemical storage device is in operation.
However, as a result of construction factors, it is not possible to ensure, despite these technical precautions, that the film of the anode material brought in this manner into contact with the solid electrolyte has a sufficiently uniform thickness distribution. In this respect, during current flow in both charging and discharging operation, nonuniformities in heat distribution sometimes occur over the region of the solid electrolyte wetted with the anode material. Depending on the filling level in the anode compartment, when the electrochemical storage device is in operational orientation, heat is in fact dissipated in the bottom region from the solid electrolyte via the liquid metal outwards to the side part, wherein in the region of the top part of the electrochemical storage device, because the anode compartment is incompletely filled with anode material, heat dissipation can only proceed via the upper region of the anode compartment otherwise still filled with gas. In the bottom region, direct thermal conduction outwards to the side part mediated by the liquid metal is thus possible, which is distinctly more favorable than the indirect thermal conduction in the top region which is mediated via a gas region.
When the electrochemical storage device is subjected to high electrical loads, undesirable temperature gradients may therefore form which lead to mechanical stresses in the solid electrolyte. Such stresses in turn have a negative impact on the service life of the electrochemical storage device. It is accordingly for example known that electrochemical storage devices exposed to particularly severe thermal loads have a distinctly shorter cycle life than those storage devices which are operated at a lower current density for charging or discharging. However, this undesirably restricts the power densities of the electrochemical storage devices which are achievable for specific applications and simultaneously also reduces the flexibility and usability of storage devices based on this technology.
For example, if one of the storage devices in a module provided with such electrochemical storage devices fails, it has been found that the majority of such failures are attributable to an electrical short circuit within the electrochemical storage device. As a consequence, the anode material may sometimes react directly with the cathode material or the potential gradient may sometimes break down, since, once it has been damaged, the solid electrolyte then permits extensive, substantially free mass exchange or an electrically conductive short-circuit connection is formed. Due to the electrochemical short circuit in the storage device, the latter is no longer in a position, for instance, to contribute a proportion of the total voltage of the module in which the storage device is interconnected, whereby the total voltage of the module falls. When individual electrochemical storage devices are connected in series in the module, this merely results in clearly foreseeable declines in the total voltage. However, when a plurality of modules are connected in parallel, mixed potentials may occur, which expose the modules already damaged by the failure of individual electrochemical storage devices to higher charging or discharging current densities, such that already existing damage in individual electrochemical storage devices may sometimes be made even worse. One consequence would be increasing failure of individual electrochemical storage devices in the module which is already damaged, i.e. provided with one or more already short-circuited storage devices, and hence as a result ultimately failure of the entire module. Another consequence would be a steady decrease in the usable storage capacity of the interconnected modules, since modules without storage devices which have failed would only be partially charged or discharged due to the mixed potentials which arise.